1. Field of the Invention
The present invention relates to a grinding method, and in particular, it relates to a workpiece grinding method which takes as an object to be ground a workpiece having a cylindrical portion and an end surface portion perpendicular thereto and which is practiced in removing a grinding allowance of a predetermined width at at least the end surface portion with a grinding wheel.
2. Discussion of the Related Art
A grinding method illustrated in FIG. 8(a) has been known as one for grinding a workpiece W such as crankshaft or the like, that is, the workpiece W having a cylindrical portion 42, end surface portions 43 (called also as flanged surface portion) perpendicular to the cylindrical portion 42 and rounded corners 44 adjoining the end surface portions 43 with the cylindrical portion 42. In the known grinding method, there is used a grinding wheel 41 having a grinding wheel layer which coincides in shape with a finished shape (indicated by the two-dot-chain line) of the workpiece W, and a plunge grinding is performed to grind the cylindrical portion 42, the end surface portions 43 and the rounded corners 44 of the workpiece W at a time.
However, in the aforementioned grinding method, since shoulder portions 45 only of the grinding wheel 41 work to grind the end surface portions 43 of the workpiece W, the grinding amount per unit area which is removed by each of the shoulder portions 45 of the grinding wheel 41 is increased, whereby the shoulder portions 45 of the grinding wheel 41 suffer local wear as indicated for example by the two-dot-chain line. As a consequence, because the shoulder portions 45 of the grinding wheel 41 are large in wear, the grinding wheel 41 has to be trued frequently though a circumferential surface portion 46 thereof remains alive to serve yet. This results in shortening the service life of the grinding wheel 41.
To overcome the aforementioned problem, there has been proposed another grinding method illustrated in FIG. 8(b). This method is implemented by using a grinding wheel 48 which is narrower in width than the cylindrical portion 42 of the workpiece W and by moving the grinding wheel 48 in the axial direction while effecting a plunge feed of the grinding in the radial direction of the workpiece W, that is, by effecting the oblique feeding as indicated by the arrow A, so that either one of the end surface portions 43, the rounded corner 44 and the external surface of the cylindrical portion 42 are ground simultaneously. In this method, it becomes possible to decrease the wear of each shoulder portion 49 because the grinding amount per unit area removed by each shoulder portion 49 is decreased. For this reason, in this latter grinding method, the local wear at each shoulder portion 49 is decreased, and therefore, it can be realized to suppress the frequent executions of the truing operation. In addition, because of the simultaneous grindings of the end surface portion 43, the rounded corner 44 and the cylindrical portion 42, it becomes possible to shorten the machining time.
However, in the latter mentioned prior art grinding method illustrated in FIG. 8(b), the performance of discharging grinding chips is deteriorated because the surface contact takes place between each end surface portion 43 of the workpiece W and the corresponding end surface portion 50 of the grinding wheel 48 and because the contact arc of the grinding wheel 48 brought into contact with the end surface portion 43 is lengthened in the rotational direction. This brings about a cause to plug pores of the grinding wheel 48 with the grinding chips and hence, to increase the grinding heat generation. In particular, in the case of the grinding heat generation being excessive, not only grinding burns but also local expansion is brought about on the workpiece W, whereby it becomes impossible to secure the perpendicularity of the end surface portion 43 to the cylindrical portion 42.
Further, the cooling performance is also lowered because the surface contact between the end surface portion 50 of the grinding wheel 48 and the end surface portion 43 of the workpiece W makes it difficult for coolant fluid reach the ground surface being heated. In other words, the deterioration in the cooling performance expedites the increase of the heat generation, so that it becomes difficult to enhance the grinding efficiency (the workpiece volume removed during a unit time period) by, for example, making the grinding speed faster. Where the truing interval is set to be shorter as alternative, it may become possible to suppress the grinding burn to some extent even in the case of a grinding operation at an enhanced grinding efficiency. However, the alternative undesirably results not only in a higher tool cost but also in work increase for the frequent grinding wheel exchanges.